1. Field of the Invention
The present invention relates to a circuit for providing a voltage signal to drive the organic light emitting diodes (OLEDs) in a pixel array display and more particularly to such a driver circuit that automatically compensates for the nonlinear voltage to luminance behavior of the pixel OLEDs due to temperature and process variability inherent in the OLED manufacturing process.
2. Description of Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Pixel drivers can be configured as either current sources or voltage sources to control the amount of light generated by the OLEDs in an active matrix display. AMOLED microdisplays require very low amounts of current to generate light, especially when using analog gray scale rendition techniques. OLEDs have typically been driven in current mode due to the linear dependence of luminance on operating current. For low light level applications, a typical OLED microdisplay pixel current is about 200 pA. Traditionally, a long channel transistor is used to generate the output current. Realizing a compact circuit that can fit in a microdisplay application precludes the use of very long channel transistors.
The relationship between the current in an OLED diode and the voltage across the diode is known as its IV characteristic. The OLED current, I, is equal to a non-linear function of the voltage across the diode, V, and can be expressed in general as I=f(V).
The inverse IV characteristic of the OLED diode is expressed as I=f−1 (V) where f−1 is a function that can be used to linearize the normal OLED IV characteristic. Using well known mathematical principles, the normal and inverse IV characteristics are related by the expression I=k*V=f−1 (f(V) where k is a constant.
Driving OLEDs in voltage mode, on the other than, has not been used in the past due to the nonlinear voltage to luminance behavior of the OLED which varies under different operating conditions. For example, the shape of the nonlinear IV characteristic will change form one display to the next due to variability in the OLED manufacturing process. The operating temperature will introduce another variability in the OLED IV characteristic which has to be taken into account during normal operation of the microdisplay. Finally, the operating voltage range of the OLED, which is used to control the average brightness of the display, will also change the shape of the required gamma curve.
A significant benefit of the voltage drive mode is the ability to miniaturize the pixel cell while still providing good control for low light applications. Very long channel transistors are not required as the drive transistor can be operated as a voltage source with good pixel to pixel uniformity. Miniaturization is a key drive to reduce the cost of AMOLED microdisplays and provides a strong incentive to implement a voltage mode of operation for next generation products. To achieve this goal, however, the problem of the variable gamma correction has to be solved.
The techniques published by OLED display designers mostly address direct view displays (displays having a diagonal greater than 2″ typically) using non-crystalline silicon processes. Those techniques were primarily developed to address the high threshold voltage variability inherent to the processes. Because of the relative large display size (when compared to microdisplays), there is no need for very low current operation and therefore none of these displays make use of the subthreshold region.
The threshold voltage compensation techniques described in the literature are of two types:                Voltage based compensation using a second storage capacitor to store the threshold voltage at each pixel        current based compensation using a technique similar to that first developed in the eMagin Corporation SVGA+ microdisplay as described in O. Prache, “Full-color SVGA+OLED-on-silicon microdisplay”, Journal of the SID, pp 133-138, 2002.        
The implementation that we are aware of that is closest to the present invention is a design from Hitachi, described in the SID symposium Proceedings of 2002, 2003 and 2004. It uses a voltage compensated pixel cell and a ramp applied to the storage element via the data line.
The primary objective of the present invention is to achieve an accurate gamma function for a voltage driven AMOLED microdisplay that is independent of manufacturing variability and operational conditions. This will allow miniaturization of the pixel structure and a size reduction of the overall microdisplay area. This invention is also applicable to larger format displays that use an active matrix architecture. The benefit is a less expensive device with improved image quality that is required for large volume application.
It is not possible to build a fixed gamma correction function into the microdisplay that can compensate for the nonlinear voltage to luminance behavior of the OLED display under all operating conditions. The nonlinear characteristic of the OLED varies from one display to all the next due to process variability and within the display due to changes in operating conditions such as temperature and bias.
In general, the present invention achieves its objective by using a Gamma reference circuit to create an output voltage signal to drive the OLED diodes into the matrix OLED display that replicates the inverse IV characteristic of the OLED device. The circuit compensates for the dependency on temperatures and process variability of the Gamma curve.
More specifically, the present invention employees an adaptive gamma correction scheme. A spare OLED diode is provided in the microdisplay which serves as a reference device for the gamma correction function. The OLED reference diode is configured to operate under the same bias and temperature conditions as the pixel diodes, but in the reverse mode. That is, a current proportional to the input signal is fed into the OLED reference device, and the voltage at the anode node provides the resulting output signal. In the pixel array, on the other hand, the input or drive signal is a voltage applied to the anode of the OLED diode and the resulting current is the output signal. The transfer function formed by the input to output signals across the spare OLED device provides the desired gamma function needed to achieve a linear input signal to output current relationship.
Since both the reference and pixel OLED devices are formed at the same time in the display, the IV characteristics will match to a high degree of accuracy. Also, the diode in the reference circuit will operate at the same current density as the pixel device because they are both tied to a common cathode voltage via the same VCOMMON line. This assures that the reference and pixel OLED devices always have the same bias conditions. As a result, as the temperature and bias conditions change for a particular microdisplay, the gamma correction function created from the reference diode will adapt its shape and will continue to produce a perfect match for the diodes used in the pixel array.